Formation Mechanisms of Secondary Inorganic Components in Fine Particulate Matter in a Typical City of the Fenwei Plain
Secondary inorganic aerosols (SNA), comprising sulfate, nitrate, and ammonium, are critical contributors to PM2.5 pollution in the Fenwei Plain, yet their formation mechanisms remain poorly characterized. Wintertime observations in Taiyuan revealed SNA as the dominant PM2.5 component, with a mean mass concentration of 33.19 ± 18.72 μg m−3, accounting for 47.13% of total PM2.5 mass. SNA concentrations increased markedly with pollution severity, but even under relatively clean conditions, SNA maintained a high mass fraction. Diurnal variation and correlation analyses indicated that nitrate formation pathways differed between day and night, largely governed by relative humidity (RH). During daytime, high RH (>55%) facilitated the partitioning of gaseous HNO3 to particulate nitrate. At night, RH positively correlated with nitrate concentration and nitrogen oxidation rate (NOR), with increased aerosol liquid water content (AWC) promoting NO2-to-nitrate conversion. The PM2.5 pH ranged from 4.3 to 5.2, and sulfate formation was primarily driven by H2O2 oxidation, with NO2 oxidation as a secondary pathway. These findings enhance understanding of SNA formation in the Fenwei Plain and provide a scientific basis for air quality policy.